Nuclear magnetic resonance (=NMR) apparatuses, in particular NMR spectrometers and NMR tomographs, require strong magnetic fields, which are often generated by using superconducting magnet coils. The superconducting magnet coils must be operated at a cryogenic temperature. For this reason, the magnet coils are typically arranged in a cryogen tank of a cryostat, which is filled with a cryogenic liquid, e.g. liquid helium. In order to maintain the operating temperature on a long-term basis and at the same time minimize the consumption of cryogenic liquids, the cooling arm of a cold head projects into the cryogen tank and heat can be withdrawn thereby. The cryogen tank is surrounded by a vacuum tank for thermal insulation.
The NMR measurements can be disturbed by mechanical vibrations of the NMR apparatus, which are introduced, in particular, via the cold head attached to the cryostat.
In the case of cooling in accordance with the pulse tube refrigerator principle, which is often applied, periodic pressure fluctuations of a working gas are produced in the cold head. For this purpose, a control valve alternately connects a high-pressure reservoir and a low-pressure reservoir of the working gas to the cold head; the switching frequency of the control valve is typically approximately 1 to 2 Hz. Disturbing vibrations at the cold head also occur in other cooling principles (e.g. Stirling, Gifford-McMahon).
In the case of cooling in accordance with the pulse tube refrigerator principle, the working gas is usually connected to the cold head via a flexible connecting line, see e.g. EP 0 780 698 A1. The flexible connecting line minimizes the introduction of high-frequency disturbances in the cold head.
The periodic pressure fluctuations in the flexible connecting line, which periodically change the length and possibly the curvature of the flexible connecting line, may, however, lead to corresponding movements of the cold head, which are introduced into the cryostat and disturb the NMR measurements.
It is known from DE 10 2014 214 819 B3 to use two line branches in a pulse tube refrigerator system to connect the control valve and cold head so that, with identical pressure fluctuations in both branches, opposing mutually compensating mechanical forces occur. Thus, application of force on the cold head can be reduced. However, the structure is relatively bulky.
DE 10 2014 219 849 B3 describes suspending the cold head at a cryogen tank wall using vibration-isolating decoupling elements. A flexible sealing element between the vacuum tank wall and the room temperature part of the cold head seals off the inside of the cryogen tank from the environment, so that only a minimum pressure difference exists on either side of the flexible sealing element. The structure reduces force transmission from the cold head to the cryogen tank.
The use of vibration-isolating decoupling elements is, however, comparably expensive and requires trained personnel for adjustment. Moreover, the size of the assembly is not negligible, making use thereof on smaller cryostats more difficult.